1. Field of the Invention
The present invention relates to a free layer structure for a spin valve sensor with a specular reflecting layer composed of ferromagnetic oxide and, more particularly, to such a free layer structure which can be made thinner while maintaining a desirable magnetic softness and magnetoresistance.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is preferably parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
In addition to the spin valve sensor the read head includes nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
The free layer structure typically employs a nickel iron layer which provides a desirable magnetic softness for the free layer. This means that the free layer has a low uniaxial anisotropy HK which promotes responsiveness of the free layer structure to signal fields from a rotating magnetic disk. When the free layer structure is highly responsive a small signal field will rotate the magnetic moment of the free layer structure which causes a change in the magnetoresistance of the spin valve sensor. It has been found that when the free layer structure also includes a cobalt iron or cobalt layer, sometimes referred to as a nanolayer, between the nickel iron layer and the spacer layer and interfacing the spacer layer that the magnetoresistance or magnetoresistive coefficient dr/R is improved. In order to obtain a desirable increase in the magnetoresistive coefficient dr/R, it has been further found that the thickness of the cobalt iron or cobalt layer should be on the order of 15 xc3x85. Unfortunately, any increase in the thickness of the cobalt iron layer reduces the softness of the free layer structure so that it is not as responsive to signal fields from the rotating magnetic disk. Accordingly, an increase in the ratio of the thickness of the cobalt iron or cobalt layer to the thickness of the nickel iron layer increases the uniaxial anisotropy HK of the free layer structure and reduces its softness so that it is less responsive to signal fields. Further, the free layer structure is less stable in returning to a zero bias position and has been found to have increased magnetostriction. The only way known to overcome the increase in uniaxial anisotropy HK of the free layer structure, because of an increase in the thickness of the cobalt iron or cobalt nanolayer, is to increase the thickness of the nickel iron layer so as to reduce the above-mentioned ratio. Unfortunately, this reduces the sensitivity of the head by lowering the linear bit density of the read head which is the number of magnetic bits which can be read linearly along a track of a rotating magnetic disk.
The present invention overcomes the aforementioned problems by providing the free layer structure with a cobalt iron oxide layer or a nickel iron oxide layer which is located between the nickel iron layer and the cobalt iron or cobalt layer. Each of the ferromagnetic oxide and cobalt iron or cobalt layers can be 5 xc3x85 thick and the nickel iron layer can be made with a minimum thickness necessary for satisfying the requirements of the read head without magnetic saturation. Accordingly, the overall thickness of the free layer structure can be reduced in thickness while maintaining a desirable magnetic softness of the free layer structure. It is believed that the reason for this improvement is that the ferromagnetic oxide layer acts as a specular reflector of conduction electrons in the spin valve sensor so that scattering of the electrons is increased and that this increase in scattering equates to an increase in the magnetoresistive coefficient dr/R. While the preferred embodiment of the invention employs the aforementioned cobalt iron or cobalt layer, in another embodiment of the invention the free layer structure employs a ferromagnetic oxide layer between the nickel iron layer and the spacer layer and interfaces the spacer layer. The invention can be employed in either a top or a bottom spin valve sensor or a dual spin valve sensor. In a dual spin valve sensor the free layer structure may include the nickel iron layer sandwiched between first and second ferromagnetic oxide layers and in a further embodiment may include the nickel iron layer and the first and second ferromagnetic oxide layers sandwiched between first and second cobalt iron or cobalt layers with the first and second cobalt iron or cobalt layers interfacing first and second spacer layers respectively.
An object of the present invention is to increase the softness of a free layer structure when a cobalt iron or cobalt layer interfaces a spacer layer for increasing the magnetoresistive coefficient dr/R.
Another object is to eliminate a cobalt iron layer in a free layer structure and substitute a layer which maintains a high magnetoresistive coefficient dr/R while maintaining a softness of the overall free layer structure so that the free layer structure is highly responsive to signal fields.
A further object is to provide a free layer structure with a reduced thickness while maintaining a high magnetoresistive coefficient dr/R and low uniaxial anisotropy HK.
Still another object is to provide a dual spin valve sensor with any of the aforementioned features set forth in the objects hereinabove.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.